a. Field of the Invention
This invention relates to a separator device for separating transfer medium from an image-bearing member. More particularly, it relates to a separator device having a separating belt disposed between an image-bearing member bearing thereon a toner image or an electrostatic latent image, such as a photosensitive medium or insulative medium, and a transfer medium to which the image on the image-bearing member is to be transferred.
B. Description of the Prior Art
A separator device for transfer medium will be described by taking as an example an electrophotographic copying apparatus using a drum-shaped photosensitive medium as an image-bearing member.
An electrophotographic copying apparatus generally comprises a rotatable photosensitive drum and various means disposed about the photosensitive drum successively in the direction of rotation thereof, including means for forming electrostatic latent image on the photosensitive drum, means for developing the electrostatic latent image, means for transferring the developed image to a transfer medium, and means for cleaning the surface of the photosensitive drum. In such an electrophotographic copying apparatus, the transfer medium is urged against the photosensitive drum as by a corona discharger or an image transfer roller during the image transfer, and at that time, the transfer medium becomes electrostatically attracted to the photosensitive drum so intensely that is cannot readily be separated from the latter. It is therefore necessary to provide some separating means and as an example thereof, a method is heretofore known whereby a surface portion of the transfer medium adjacent to the leading edge thereof is guided by a separating belt and separated from the photosensitive drum by a separating roll or the like. This method is highly reliable and preferable. The belt used for such separating means may preferably be formed of a sheet of tough resin material for contact with the photosensitive drum, whereas not only when it is used in its stationary position but also when it is in the form of an endless belt movable round, it is unavoidable for such belt to be in continuous contact with the photosensitive drum or the separating roll.
The separator device in the above-described apparatus of the prior art will now be described by reference to FIGS. 1 and 2. The separator device shown there comprises an endless separating belt stretched between the photosensitive drum and the transfer medium and movable at the same velocity as that of the transfer medium and in the same direction of movement as that of the latter. In FIG. 1 which is a side view of the conventional separator device, reference numeral 1 designates the photosensitive drum having a photosensitive layer 1a formed on the peripheral surface thereof and movable in the direction of arrow. In FIG. 1, only the section for the image transfer step and the section for the separating step are shown. There is further seen an image transfer corona discharger 2 and a transfer medium guide 4 for guiding a transfer medium P between the photosensitive drum 1 and the image transfer discharger 2. In the device of FIG. 1, when the toner image on the surface of the photosensitive drum 1 is transferred to the transfer medium P, the transfer medium is electrostatically attracted to the photosensitive drum 1 by the corona discharge and this is the reason why the separator device is required. In the device of FIG. 1, therefore, a separating belt 5 is provided which extends from upstream of the image transfer discharger 2 and passes in front of the discharger 2 toward a rotating separating roller 6 disposed immediately downstream of the image transfer station. The separating belt 5 is in an endless form and is designed to guide a widthwise side edge portion of the transfer medium P without bringing it into contact with the photosensitive drum 1 before image transfer takes place, and to guide the transfer medium away from the photosensitive drum 1 with a predetermined tension after the transfer medium has passed through the position of the image transfer roller, thereby separating the image transfer medium from the photosensitive drum 1. Designated by 7 is a roller disposed at the start end position of the separating belt 5 for imparting follow-up movement to the belt, 8 is a driving roller for imparting frictional rotation to the separating belt 5, and 9 is a guide roller for imparting tension to the separating belt 5, the guide roller 9 being connected to a spring 10 and normally biased thereby to create a tension. The separating belt 5 is moved in the direction of arrows substantially at the same velocity as the transfer medium P as long as the transfer medium P is passing, but one side edge of the separating belt 5 is then in frictional state with respect to the photosensitive drum 1 and therefore, the separating belt 5 should preferably be formed of a material which is low in friction coefficient.
The driving roller 8 should preferably have as large a friction coefficient as possible, like a rubber-coated roll, for example, in order to move around the separating belt 5 without slipping. Designated by 11 is a belt for transporting the transfer medium P to the fixing station, 12 a guide between the separating roller 6 and the belt 11, and 13 a blower which, if required, may be provided for imparting an air flow. When fed through transport rollers 3 and guide 4 and between the photosensitive drum 1 and the image transfer discharger 2, the photosensitive medium P is electrostatically urged against the photosensitive drum 1 by the image transfer discharger 2, whereby transfer of the toner image is effected with the side edge portion of the transfer medium P urged against the photosensitive drum 1 with the separating belt 5 intervening therebetween, namely, with the side edge portion of the transfer medium being forcibly kept off the photosensitive drum while being guided along the lower side of the belt 5 toward the separating roller 6. Then, the transfer medium is guided along the roller 6 and away from the photosensitive drum 1, so that the side edge portion of the transfer medium P is separated from the photosensitive drum 1 as soon as it passes through the position of the image transfer roller, and the side edge portion of the transfer medium becomes greatly separated from the photosensitive drum 1 as the transfer medium is advanced. Thus, the separating action is gradually propagated from the separating belt side toward the widthwisely inner part of the transfer medium, until at least the transfer medium is naturally and reliably separated from the photosensitive drum 1.
Accordingly, as the angle of contact between the separating belt 5 and the separating roller increases, the separating roller 6 should preferably be closer to the transfer discharger 2 as much as possible. Once separated from the photosensitive drum 1, the transfer medium P hangs down onto the separating roller 6 and is guided to the belt 11 through the guide 12 by the friction of the transfer medium with the roller 6. FIG. 2 shows, in perspective view, the separating device of FIG. 1. In the device using the above-described separating belt, the pressure contact of the belt 5 with the photosensitive drum 1 or the separating roller 6 is unavoidable. However, when compared with the case where the separating belt 5 is fixedly stretched, the shown arrangement in which the belt 5 is moved can prevent a specific portion of the belt 5 from being damaged by friction. Nevertheless, even if the belt is of the non-endless type which is moved around, the length of the belt is limited and the durability of the belt is necessarily limited. Of course, if the separating belt is damaged, the separating function of the device will be stopped and the copying operation will be left stopped until the belt is replaced or repaired, and this will economically be very disadvantageous.
In the device of the prior art, as has been noted above, the durability of the separating belt itself is improved by the manner in which the separating belt is stretched, whereby the separating device can be operated for a long time. Nowadays, however, the increased life of the photosensitive drum makes it more and more necessary in the separating device employing the separating belt to prevent the photosensitive drum from being damaged by the separating belt. For this reason, the surface hardness of the photosensitive drum is increased as much as possible and the material for the separating belt which contacts or slidingly contacts such surface of the drum is usually selected on that side of the belt which abrades more easily with respect to the drum surface hardness. In other words, the separating belt may preferably be regarded as an expendable rather than as a permanent article. In this case, however, the life of the separating belt is shorter than the life of the copying apparatus and the separating belt must be periodically replaced, and this entails various problems such as the setting of the belt during the replacement and the procedures involved during the replacement.
Thus, in the separating belt which contacts the photosenstive drum, selection of the material therefore is important and if the separating belt is endowed with durability and wear resistance, the surface portion of the photosensitive drum contacted by the separating belt will undesirably be roughened due to the friction with respect to the belt. If, conversely, a soft material is selected for the belt, the surface and/or end portion of the belt will suffer from deterioration which will lead to the damage as described above. For this reason, the separating belt must be replaced before it is damaged at the end of its life.